#include "stdio.h"

int main() {
	long i, sign = 1;
	float sum = 0;
	i = 1;
	while (i <= 1000000000) {
		if (i % 2 == 1) {
			sum = sum + 1.0 / (sign * i);
			sign = -sign;
		}
		i++;
	}
	printf("sum=%f", 4 * sum);
	return 0;
}